1. Field of the Invention
The present invention relates to a unit for detecting a ground fault or an insulated condition of an ungrounded power source with respect to a ground potential portion, and particularly, to an apparatus for detecting a fault of a flying capacitor used in such a unit.
2. Description of Related Art
A vehicle that uses electricity as driving energy usually employs a high-voltage (for example, 200 V) DC power source configured as an ungrounded power source insulated from a vehicle body. In order to detect a ground fault or an insulated condition of the ungrounded power source with respect to the vehicle body, i.e., a ground potential portion, a detecting unit employing a flying capacitor is used.
The detecting unit controls internal switches to charge the flying capacitor with a charge amount corresponding to a voltage of the DC power source insulated from the ground potential portion, a charge amount corresponding to a ground fault resistance on the positive terminal side of the DC power source, and a charge amount corresponding to a ground fault resistance on the negative terminal side of the DC power source, respectively. Thereafter, a controller of the detecting unit measures a charge voltage at each of the charge amounts, calculates the ground fault resistances on the positive and negative terminal sides of the DC power source, and detects a ground fault or an insulated condition of the DC power source.
In recent years, the flying capacitor is required to be made of a ceramics capacitor that is small and realizes large capacitance. The ceramics capacitor is known to greatly change the capacitance thereof depending on a DC bias. To eliminate the influence of a DC bias on the capacitance of a ceramics capacitor, Japanese Unexamined Patent Application Publication No. 2009-281986, for example, discloses a technique of configuring the detecting unit so that a charge voltage of the ceramics capacitor becomes equal to a charge voltage occurring at the charge amount corresponding to the voltage of the DC power source in a case where a ground fault resistance is at an alarming level.